Cutting elements such as shear cutters for drag bit type of rock bits, for example, typically have a body (or substrate), which has a contact face. An ultra hard layer is bonded to the contact face of the body by a sintering process to form a cutting layer (sometimes referred to as a “cutting table”). The body is generally made from tungsten carbide-cobalt (sometimes referred to simply as “tungsten carbide” “or carbide”), while the ultra hard material layer is a polycrystalline ultra hard material, such as polycrystalline diamond (“PCD”) or polycrystalline cubic boron nitride (“PCBN”).
Common problems that plague cutting elements having an ultra hard material layer, such as PCD or PCBN bonded on a carbide substrate are chipping, spalling, partial fracturing, cracking or exfoliating of the cutting table. These problems result in the early failure of the ultra hard layer and thus, in a shorter operating life for the cutting element. Typically, these problems may be the result of peak (high magnitude) stresses generated on the ultra hard layer at the region in which the layer makes contact with an external body, such as when the cutting layer makes contact with the earthen formation during drilling.
Generally, shear cutter type cutting elements are mounted onto a drag bit body at a negative rake angle. Consequently, the region of the cutting element that makes contact with the earthen formation includes a portion of the ultra hard material layer upper surface circumferential edge. This portion of the layer is subjected to the highest impact loads. Accordingly, much of the research into shear cutters has focused on making a more durable ultra hard material layer, or making a better interface between the ultra hard material layer and the substrate. However, it is equally important that the substrate of the cutting element be durable. For example, cracks initiated in the ultra hard material layer due to contact loads can propagate into the substrate. Accordingly, the toughness of the substrate plays a significant role on the breakage resistance of cutting elements.
One common substrate material is cemented tungsten carbide. Cemented tungsten carbide generally refers to tungsten carbide (“WC”) particles dispersed in a binder metal matrix, such as iron, nickel, or cobalt. Cemented tungstem carbide having tungsten carbide particles dispensed in cobalt is often referred to as a “WC/Co” system. Tungsten carbide in a cobalt matrix is the most common form of cemented tungsten carbide, which is further classified by grades based on the grain size of WC and the cobalt content.
Tungsten carbide grades are selected primarily based on two factors that influence the lifetime of a tungsten carbide substrate: wear resistance and toughness. Existing substrates for shear cutters are generally formed of cemented tungsten carbide particles (with grain sizes in the range of about 1 to 3 μm as measured by ASTM E-112 method) and cobalt (with the cobalt content in the range of about 9% to 16% by weight), and have a hardness in the range of about 86 Ra to 89 Ra.
For a WC/Co system, it is typically observed that the wear resistance (i.e., hardness) increases as the grain size of tungsten carbide or the cobalt content decreases. On the other hand, the fracture toughness increases with larger grains of tungsten carbide and greater percentages of cobalt. Thus, fracture toughness and wear resistance tend to be inversely related, i.e., as the grain size or the cobalt content is decreased, wear resistance of a specimen is improved, and its fracture toughness decreases, and vice versa. Due to this inverse relationship between fracture toughness and wear resistance (i.e., hardness), the grain size of tungsten carbide and the cobalt content are selected to obtain a desired wear resistance and toughness.
Despite these counter-balancing concerns, conventional cutting element designs have generally focussed only on the toughness of the chosen material. For example, generally one skilled in the art would select a carbide grade with high toughness, because in earth boring applications wear of the carbide is not a major issue.
In addition, the thermal properties of a tungsten carbide substrate, such as thermal conductivity, are generally not considered. As a result, thermal fatigue and heat checking in tungsten carbide substrates are issues that have not been adequately resolved. Consequently, substrates made of conventional tungsten carbide grades frequently fail due to heat checking and thermal fatigue when subjected to high temperature and high loads.
Accordingly, there exists a need for improving the toughness of carbide substrate without significantly reducing the wear resistance and thermal conductivity.